1. Field of the Invention
The present invention relates to a structure in which a laser beam which has been processed into a linear beam is scanned and irradiated. The present invention can be used in a process for manufacturing a semiconductor device using the emission of a laser beam, an exposing process using the irradiation of a laser beam, and so on.
2. Description of the Related Art
In recent years, there has been studied a technique of forming an amorphous semiconductor film or a crystalline semiconductor film (a semiconductor film having crystallinity such as polycrystal or microcrystal of non-monocrystal) on an insulating substrate made of glass or the like.
In this technical field, a technique has been widely researched in which laser annealing is conducted on an amorphous silicon film or a low-crystalline silicon film to crystalize the film or to improve crystallinity.
The glass substrate is inexpensive and rich in processing property, in comparison with a quartz substrate which has been conventionally frequently used, as a result of which it is advantageous in that a large-area substrate can be readily fabricated. This is a reason why the above study has been made. Also, the reason why the layer is frequently used for crystallization is that a melting point of the glass substrate is low. The laser can give a high energy to only the non-monocrystal film without largely changing the temperature.
Because a crystalline silicone film formed by laser annealing is high in mobility, a thin film transistor (TFT) is formed using the crystalline silicon film
The above technique enables a monolithic liquid crystal electro-optic device where TFTs for pixels and drive circuits are disposed on a single glass substrate to be obtained.
A crystalline silicon film obtained by laser annealing are formed of a large number of crystal grains, and therefore is called "polycrystal silicon film" or "polycrystal semiconductor film".
In the above-described laser annealing technique, since a laser beam needs to be irradiated onto an area 10 cm square or more, the irradiating method must be devised.
There have been proposed some methods of irradiating a laser beam, for example:
(1) A laser beam is converted into a square spot of several cm square on a plane to be irradiated and irradiated thereon while it is scanned. PA1 (2) A laser beam is processed through an optical system so as to be converted into a linear beam of several mm width x several tens cm length, and such a linear laser beam is irradiated on the plane while it is scanned. PA1 (1) A line formed by the collection of the focal points and an orientation vector of an incident laser beam are not orthogonal to each other. PA1 (2) A plane that includes a line containing a direction from which the sectional configuration 1103 is extended and a line formed by the collection of the focal points are orthogonal to the plane of the sectional configuration 1103. PA1 (3) The angle Y is not a right angle.
Of those methods, in the method (1), portions where laser beams as irradiated are overlapped are increased, thereby being liable to make the irradiation effect uneven. Also, the productivity is low.
On the other hand, in the method (2), the irradiation unevenness is difficult to exhibit in comparison with the method (1), and the productivity is also high.
Particularly in the method (2), the use of the linear laser beam is different from the use of a spot-like laser beam requiring scanning in the front and rear direction and in the right and left direction in that the laser can be irradiated on the entire plane to be irradiated by scanning only in a direction (width direction) perpendicular to the linear direction (longitudinal direction) of the linear laser, the high productivity can be obtained. The reason that scanning is made in the direction perpendicular to the linear direction is that it is the scanning direction where the coefficient is the highest. Because of the high productivity, the use of the linear laser beam is a main stream for laser annealing at present.
However, there arise some problems when laser annealing is conducted on the non-monocrystal semiconductor film while scanning a pulse laser beam which has been processed into a line.
In particular, one of the severe problems is that laser annealing is not uniformly conducted on the entire film surface.
When the linear laser started to be used, a phenomenon that stripes are produced on portions where the adjacent beams are overlapped with each other was remarkable, and the semiconductor characteristic of the film was largely different depending on each of the stripes.
What is shown in FIG. 1A is a picture that photographs a surface state of a crystalline silicon film obtained by scanning a linear laser beam, which is longitudinal along a lateral direction of a paper surface, in its width direction (a vertical direction of the paper surface) and irradiating it on the crystalline silicon film.
As is apparent from FIG. 1A, the degree of overlapping the linear laser beam is reflected on the crystallinity, to thereby exhibit a striped pattern.
In the case of fabricating a liquid crystal display unit using a silicon film exhibiting the stripped pattern, for example, there occurs a disadvantage that the stripes appear as they are.
It is presumed that this results from reflecting a difference in crystallinity of the striped pattern on the characteristic of a TFT array.
The above problem can be greatly improved by improving a non-monocrystal semiconductor film on which a laser beam is to be irradiated, thinning the scanning pitches (intervals between the respective adjacent linear laser beams) of the linear laser beam, optimizing a condition under which the linear laser beam is scanned, or other manners. More specifically, in the application of a liquid crystal display device, the degree of overlapping the linear laser beams on each other can be restrained from directly adversely affecting an image quality.
However, subsequent to the solving of the problem caused by the overlapped pulse laser shots, the nonuniformity of the energy distribution of the beam per se has been remarkable.
In general, in the case of forming the linear laser beam, an original rectangular (or square, or circle) beam is processed into a line through an appropriate optical system.
The original rectangular beam is about 2 to 5 in aspect ratio. That original beam is deformed into a linear beam 100 or more in aspect ratio through an optical system. For example, it is deformed in a linear laser beam 1 mm in width and 200 mm in length.
The formation of the laser beam is devised such that an energy distribution within the laser beam gets uniform in quality. In particular, employing an optical system called "beam homogenizer," the energy density within the laser beam is made uniform.
An outline of the structure of a device for irradiating a linear laser beam is shown in FIG. 2. In FIG. 2, there are shown a laser oscillator indicated by reference numeral 201, a beam homogenizer consisting of lenses 202, 203, 204 and 205, a mirror 206 and an objective lens 207.
In this example, the combination of the lenses 203 and 205 is a beam homogenizer for improving the energy distribution in a longitudinal direction of the linear laser beam.
Also, the combination of the lenses 202 and 204 is a beam homogenizer for improving the energy distribution in a width direction of the linear laser beam.
The action of the beam homogenizer is that the original longitudinal beam is divided into a plurality of beams which are then enlarged, respectively, and re-superimposed on each other.
Seemingly, the beam divided and re-constructed by the beam homogenizer becomes more uniform in the distribution of energy as the division becomes more fine.
However, in fact, when the beam is irradiated onto the semiconductor film, the stripped pattern shown in FIG. 1B appears regardless of the fineness of the division.
The stripped patterns are innumberably formed so as to be orthogonal to the longitudinal direction of the linear laser beam. The formation of such stripped pattern is caused by the stripped distribution of the energy of the original rectangular beam or optical system.
The present inventor has conducted a simple experiment to make sure of the cause why the above-described stripes are formed.
This experiment has been made to investigate how the above striped pattern is changed by rotating a laser beam in a state where the original rectangular laser beam is entered into the optical system, that is, the laser beam outputted from the oscillator.
As a result of the experiment, the vertical stripes are not changed at all. It has been found, therefore, that the formation of the stripped pattern is made by not the original rectangular beam but the optical system.
It can be explained that since this optical system shown in FIG. 2 is designed to divide and re-couple the beam which has a single waveform and is in phase (since the laser obtains the intensity by making the phase coincide with each other, the laser beam is in phase) to unify the distribution of the energy, the stripes are the interference stripes of a light.
The interference of light is a phenomenon which is caused by a phase shift when lights in phase and identical in wavelength are superimposed on each other with an optical path difference, that is, by mutually cyclically strengthening or weakening the light.
FIG. 3 schematically shows an appearance where lights that have passed through five slits 301 are interfered with each other. FIG. 3 shows whether the lights entered from the left side of the slits are also interfered with each other on the right side of the slits, or not, with a light intensity I as a parameter.
In the case where the five slits 301 are located at regular intervals, a center of an interference peak occurs on a portion corresponding to a center A of the slit group.
Interference stripes are formed with that peak as a center.
Let us consider that the principle of the occurrence of the interference stripes shown in FIG. 3 is applied to a cylindrical lens group 401 and a cylindrical lens 402 shown in FIG. 4.
The cylindrical lens group 401 shown in FIG. 4 corresponds to the cylindrical lens group 203 shown in FIG. 2. Also, the cylindrical lens group 402 shown in FIG. 4 corresponds to the cylindrical lens group 205 shown in FIG. 2.
Also, portions A, B and C in FIG. 4 correspond to portions A, B and C in FIG. 3.
In FIG. 4, the number of divisions of beams made by the cylindrical lens group 401 corresponds to the number of slits shown in FIG. 3.
In case of the structure shown in FIG. 4, peaks of interferences (mutually strengthened portions) occur on the respective portions A, B and C in accordance with the principle shown in FIG. 3.
The actual interference stripes made by a laser do not exhibit distinct intensity with a complete periodic property. It is presumed that this phenomenon results from a slight optical shift in the optical system or the dispersion of an energy caused by heat conduction in a semiconductor film.
In FIG. 2, since the combination of the cylindrical lens group 202 and the cylindrical lens 204 gives the entire same action as that of the combination of the cylindrical lens group 203 and the cylindrical lens 205 to the laser beam, the same interference of light occurs in the width direction of beam within the linear laser beam.
However, since the light interference in the width direction of the linear laser beam occurs in an area several mm or less in width, it is almost inconspicuous. That is, there does not particularly arise any problems.
The above interference state of light within the linear laser beam is schematically shown in FIG. 6. In FIG. 6, reference numeral 601 denotes a plane onto which the linear laser beam is irradiated. Also, reference numeral 602 denotes a portion where the peak of interference is high.
Schematically as shown in FIG. 6, a peak 602 of interference distributes in a lattice manner in the region onto which the linear laser beam is irradiated. As described above, however, the peaks of interference in the width direction of the linear laser beam are almost inconspicuous.
In general, the distribution of the peaks of interference does not have uniform intervals. This is because the linear beam is obtained by linearly synthesizing spherical waves (If a spherical waver is straightly cut, the intervals of the in-phase stripes are not constant.).
In the case where the intervals of peaks of interferences are intended to be substantially constant, the plane waves may be synthesized into a linear wave using the optical system shown in FIG. 5 (if the plane wave is straightly obliquely cut, the intervals of the in-phase stripes are constant.).
A difference of the optical system shown in FIG. 5 from that of FIG. 4 resides in that laser beams divided by the cylindrical lens group 501 on the beam incident side is processed into a parallel light ray by a subsequent cylindrical lens 502.
The optical system of this type can be simply obtained by appropriately selecting a distance between the forward cylindrical lens group 401 and the backward cylindrical lens 402 in FIG. 4.
With the above arrangement, any beams divided by the cylindrical lens group 501 are processed into the plane waves by the cylindrical lens 502. When the beams processed by this optical system is used, the intervals of the vertical stripes become substantially constant.
As shown in FIG. 6, the linear laser beam exhibits the distribution of peaks of interference in a lattice manner within the beam.
Therefore, when the laser beam is scanned in the width direction of the linear laser beam and irradiated on the plane, a light beam strong or weak in the intensity of interference within the beam is caused to be repeatedly irradiated onto the same portion of an object to be irradiated.
As a result, stripes caused by the strong intensity or the weak intensity are formed along the beam scanning direction. That is, the striped pattern appears along the scanning direction of the laser beam.
The above-described striped pattern permits the peaks of light interference which distribute in the width direction perpendicular to the linear direction of the linear laser to be particularly emphasized by overlapping the linear lasers with each other at pitches sufficiently thinner than the beam width.
An appearance where stripes vertically transverse to the linear laser are formed is schematically shown in FIG. 7. A linear laser beam 701 periodically exhibits the intensity of an energy caused by the light interference in its linear direction (As already discussed in the above, the linear laser beam exhibits the intensity of periodic energy due to the light interference also in its width direction, but hardly adversely affects the present invention.
As shown in FIG. 7, the stripes are caused to be emphasized by overlapping those laser beams with each other.
In order to prevent the striped pattern from being emphasized as shown in FIG. 7, it is greatly effective that the linear laser beams are obliquely overlapped with each other as shown FIG. 8. This technique is disclosed in Japanese Patent Application Laid-open No. Hei 9-61781 of which application has been filed by the present applicant.
With the above arrangement, the peak portions of interference are distributed without being applied to the same location several times, thereby being capable of restraining the formation of the striped pattern.
However, the processing method shown in FIG. 8 suffers from such a problem hat the length of a laser beam cannot be utilized at the maximum.